I think its this code that causes the problem.

		if (preg_match('#(\d+)\.(\d+)\.\d+[a-z]?#', $version, $matches))

If you have your mod's modx files (install or upgrade in contrib folder) contain a version such as 1.0.RC1 it will fail. phpBB itself used the same type of versioning with phpBB 3. So this should not fail.